After calibrating, I connected my nanovna to my VHF antenna via a brand new 75 foot length of 50ohm DX400MAX coax. The SWR measurement oscillates up and down every 4MHz or so (see attached). These correspond exactly to the half-wavelength harmonics of the cable. Looking at the smith chart, it is clear that the impedance is tracing a circle around about 45 ohms rather than the constant-SWR circle centered at 50 ohms, resulting in a frequency-dependent SWR.
Is there is a good way to determine whether this is a problem with my nanovna, with the calibration standard, or with the cable?
One note: the calibration standard load measures 51 ohms DC resistance, which seems like a problem to me. Should my standard be exactly 50 ohms dc resistance?
